Wireless communication networks often use digital modulation schemes, the standards for which carry stringent requirements on out-of-band emissions. In addition, regulatory bodies, such as the Federal Communications Commission (FCC), for example, also limit out-of-band emissions.
One source of out-of-band emissions and/or undesired emissions in-band, collectively referred to herein as spurious emissions, are the RF power amplifiers used to transmit signals to mobile units, such as in a cellular network. Spurious emissions are often due to non-linearities, e.g., amplitude-to-amplitude (AM-AM) and amplitude-to-phase (AM-PM), as well as memory effects, in the response of the RF power amplifiers.
One manner of addressing these concerns is to use a technique referred to as “predistortion” to apply a correction to the input signals of these amplifiers to reduce non-linearities and the spurious emissions that result therefrom. However, conventional predistortion techniques often suffer from certain drawbacks, limiting their utility.
For example, RF based predistorters often contain a number of delay paths, each associated with a look-up table containing correction values, that may be used to provide some portion of a correction signal used to reduce non-linearities and memory effects in the response of the RF power amplifier. Further, these delay paths may be organized into layers. The first of these layers may be configured to provide AM-AM and AM-PM or memory-less distortion, while the second layer corrects for memory effects.
The correction values in the look-up tables may be initially generated for a particular operating frequency or channel for the RF power amplifier. So long as the RF power amplifier is used at this frequency, the correction values in look-up tables in their respective layers remain independent and a cancellation null occurs at the operating frequency, thus preventing a portion of the carrier frequency from appearing in the second layer RF paths. Further, the first and second layers may be turned on and off independently to determine the effectiveness of each layer, and to allow alignment and/or tuning.
However, if the RF power amplifier is used at some other frequency, a portion of the carrier frequency may appear in the second layer RF paths and result in a spurious emission. In order to prevent such a spurious emission, the correction values in the look-up tables in the first and second layers may be adjusted. Such an adjustment makes the correction values in the look-up tables in each layer interdependent. The interdependence of the correction values in the look-up tables in each layer generally prevents independent operation of each layer to allow for independent tuning and/or alignment.
Generation of the correction values in the look-up tables also becomes an iterative process when the correction values are interdependent. For example, in order to minimize spurious emissions in an interdependent system, correction values in the look-up table associated with the first layer are generated while the second layer is turned off. Next, the first layer is turned off while correction values for the look-up tables in the second layer are generated. Multiple iterations between the first and the second layers are often required to eliminate the presence of the carrier in the second layer RF paths, reducing any spurious emissions associated therewith. This process is repeated until the spurious emissions are eliminated or reduced to a satisfactory level.
In addition, the further the operating frequency is from the frequency used to generate the initial correction values in the look-up tables, the more iterative the process becomes. Thus, a frequency located at the center of the operating band of the RF power amplifier is typically selected to generate the initial correction values. RF power amplifiers used for digital modulation schemes typically have sufficient operating bandwidth to make generating correction values in the look-up tables in each layer difficult even when a frequency located at the center of the band is selected for initial generation of correction values. More particularly, those frequencies located at the upper and lower extremes of the operating band are typically the most difficult to generate correction values for, e.g., requiring a high number of iterations, and making the correction values in the look-up tables in each layer even more interdependent.
There is therefore a need to provide a predistorter capable of reducing spurious emissions in the response of an RF power amplifier with independent delay paths irrespective of the operating frequency of the RF power amplifier.